The Plasma Magnet Drive: A Simple, Cheap Drive for the Solar System and Beyond

byPaul GilsteronDecember 29, 2017

Can we use the outflow of particles from the Sun to drive spacecraft, helping us build the Solar System infrastructure we’ll one day use as the base for deeper journeys into the cosmos? Jeff Greason, chairman of the board of the Tau Zero Foundation, presented his take on the idea at the recent Tennessee Valley Interstellar Workshop. The concept captured the attention of Centauri Dreams regular Alex Tolley, who here analyzes the notion, explains its differences from the conventional magnetic sail, and explores the implications of its development. Alex is co-author (with Brian McConnell) of A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer, 2016), focusing on a new technology for Solar System expansion. A lecturer in biology at the University of California, he now takes us into a different propulsion strategy, one that could be an enabler for human missions near and far.

by Alex Tolley

Suppose I told you that a device you could make yourself would be a more energy efficient space drive than an ion engine with a far better thrust to weight ratio? Fantasy? No!

Such a drive exists. Called the plasma magnet, it is a development of the magnetic sail but with orders of magnitude less mass and a performance that offers, with constant supplied power, constant acceleration regardless of its distance from the sun.

At the recent Tennessee Valley Interstellar Workshop (TVIW), Jeff Greason presented this technology in his talk [1]. What caught my attention was the simplicity of this technology for propulsion, with a performance that exceeded more complex low thrust systems like ion engines and solar sails.

What is a plasma magnet?

The plasma magnet is a type of magsail that creates a kilometers wide, artificial magnetosphere that deflects the charged solar wind to provide thrust.

Unlike a classic magsail [9] (figure 1) that generates the magnetic field with a large diameter electrical circuit, the plasma magnet replaces the circular superconducting coil by inducing the current flow with the charged particles of the solar wind. It is an upgraded development of Robert Winglee’s Mini-Magnetospheric Plasma Propulsion (M2P2) [7, 8], a drive that required injection of charged particles to generate the magnetosphere. The plasma magnet requires no such injection of particles and is therefore potentially propellantless.

Figure 1. A triple loop magsail is accelerated near Jupiter. Three separate boost beams transfer momentum to the rig, carefully avoiding the spacecraft itself, which is attached to the drive sail by a tether. Artwork: Steve Bowers, Orion’s Arm.

Developed by John Slough and others [5, 6], the plasma magnet drive has been validated by experimental results in a vacuum chamber and was a NIAC phase 1 project in the mid-2000s [6]. The drive works by initially creating a rotating magnetic field that in turns traps and entrains the charged solar wind to create a large diameter ring current, inducing a large scale magnetosphere. The drive coils of the reference design are small, about 10 centimeters in diameter. With 10 kW of electric power, the magnetosphere expands to about 30 kilometers in diameter at 1 AU, with enough magnetic force to deflect the solar wind pressure of about 1 nPa (1 nN/m2) which produces a thrust in the direction of the wind of about 1 newton (1N). Thrust is transmitted to the device by the magnetic fields, just as with the coupling of rotation in an electric motor (figure 2).

For a fixed system, the size of the induced magnetosphere depends on the local solar wind pressure. The magnetosphere expands in size as the solar wind density decreases further from the sun. This is similar to the effect of Janhunen’s electric sail [2] where the deflection area around the charged conducting wires increases as the solar wind density decreases. The plasma magnet’s thrust is the force of the solar wind pushing against the magnetosphere as it is deflected around it. It functions like a square-rigged sail running before the wind.

Figure 2. Plasma magnetic sail based on rotating magnetic field generated plasma currents. Two polyphase magnetic coils (stator) are used to drive steady ring currents in the local plasma (rotor) creating an expanding magnetized bubble. The expansion is halted by solar wind pressure in balance with the magnetic pressure from the driven currents (R >= 10 km). The antennas (radius ~ 0.1 m) are shown expanded for clarity. [6]

The engine is little more than 2 pairs of charged rotating coils and is therefore extremely simple and inexpensive. The mass of the reference engine is about 10 kg. Table 1 shows that the plasma magnet has an order higher thrust to weight ratio than an ion engine and 2 orders better than a solar sail. However, as the plasma magnet requires a power source, like the ion engine, the comparison to the solar sail should be made when the power supply is added, reducing is performance to a 10-fold improvement. [ A solar PV array of contemporary technology requires about 10 kg/kW, so the appropriate thrust/mass ratio of the plasma magnet is about 1 order of magnitude better than a solar sail at 1 AU]

The plasma magnet, as a space drive, has much better thrust to weight ratio than even the new X-3 Hall Effect ion engine currently in development. This ratio remains high when the power supply from solar array is added. Of more importance is that the plasma magnet is theoretically propellantless, providing thrust as long as the solar wind is flowing past the craft and power is supplied.

Name

Type

Thrust/weight (N/kg)
Engine mass only

Thrust/weight (N/kg) with power supply

SSME

Chemical

717

N/A

RD-180

Chemical

769

N/A

plasma magnetosphere

Electro-magnetic

0.1

.01

NSTAR-1

Ion (Gridded)

0.004

0.002

X-3

Ion (Hall Effect)

0.02

0.004

Solar Sail

Photon Sail

0.001 (at 1 AU)

N/A

Table 1. Comparison of thrust to mass ratios of various types of propulsion systems. The power supply is assumed to be solar array with a 10 kg/kW performance.

The downside with the plasma magnet is that it can only produce thrust in the direction of the solar wind, away from the sun, and therefore can only climb up the gravity well. Unlike other propulsion systems, there is little capability to sail against the sun. While solar sails can tack by directing thrust against the orbital direction, allowing a return trajectory, this is not possible with the basic plasma magnet, requiring other propulsion systems for return trips.

Plasma magnet applications

1. Propulsion Assist

The most obvious use of the plasma magnet that can only be used to spiral out from the sun is as a propellantless assist. The drive is lightweight and inexpensive, and because it is propellantless, it can make a useful drive for small space probes. Because the drive creates a kilometers sized magnetosphere, scaling up the thrust involves increased power or using multiple drives that would need to be kept 10s of kilometers apart. Figure 3 shows a hypothetical gridded array. Alternatively, the plasma magnets might be separated by thrusters and individually attached to the payload by tethers.

Figure 3. Plasma magnets attached to the nodes in a 2D grid could be used to scale up the thrust. The spacecraft would be attached by shroud lines as in a solar sail with a trailing payload. Scaling up the power supply to create a larger magnetosphere is also possible.

For a mixed mode mission, the plasma magnet engine is turned on for the outward bound flight, with or without the main propulsion system turned on. The use of power to generate thrust without propellant improves the performance of propellant propulsion systems where the accumulated velocity exceeds the performance cost of the power supply mass or reduced propellant. For an ion engine as the main drive, the plasma magnet would use the same power as 4 NSTAR ion engines but provide 3x the thrust.

2. Moving Asteroids for Planetary Defense

The propellantless nature of the plasma magnet drive makes it very suitable for pushing asteroids for planetary defense. Once turned on, the drive provides steady thrust to the asteroid, propelling it away from the sun and raising its orbit. Because the drive does not need to be facing any particular direction, it can be attached to a tumbling asteroid without any impact on the thrust direction.

3. Charged particle radiation shield for crewed flights

The magnetosphere generated by the engine makes a good radiation shield for the charged particles of the solar wind. It should prove to be a good solution for the solar wind, solar flares and even coronal mass ejections (CME). This device could, therefore, be used for human flight to reduce radiation effects. For human crewed flights, the 1N of thrust is insufficient for the size of the spacecraft and would have a marginal propulsion compared to the main engines. Given the plasma magnet’s small size and mass, and relatively low power requirements, the device provides a cost-effective means to protect the crew without resorting to large masses of physical shielding. The plasma magnet would appear to be only effective for the charged solar wind, leaving the neutral GCRs to enter the craft. However, when an auxiliary device is used in the mode of aerobraking, the charge exchange mechanism should reduce the galactic cosmic ray (GCR) penetration (see item 8 below).

4. Asteroid mining

The plasma magnet thruster might be a very useful part of a hybrid solution for automated mining craft. The hybrid propulsion would ally the plasma magnet thruster with a propellant system, such as a chemical or ion engine. The outward bound trip would use the plasma magnet thruster to reach the target asteroid. The propellant tanks would be empty saving mass and therefore improving performance. The propellant tanks would be filled with the appropriate resource, e.g. water for an electrothermal engine, or for a L2/O2 chemical engine. This engine would be turned on for the return trip towards the inner system. The reverse would be used for outward bound trips to the inner system

5. Interstellar precursor using nuclear power

A key feature of the plasma magnet is that the diameter of the magnetosphere increases as the density of the solar wind decreases as it expands away from the sun. The resulting expansion exactly matches the decrease in density, ensuring constant thrust. Therefore the plasma magnet has a constant acceleration irrespective of its position in the solar system.

As the solar wind operates out to the heliopause, about 80 AU from the sun, the acceleration from a nuclear powered craft is constant and the craft continues to accelerate without the tyranny of the rocket equation. Assuming a craft with an all up mass of 1 MT (700 kg nuclear power unit, 10 kg engine, and the remaining in payload), the terminal velocity at the heliopause is 150 km/s. The flight time is 4.75 years, which is a considerably faster flight time than the New Horizons and Voyager probes.

Slough assumed a solar array power supply, functional out to the orbit of Jupiter at 5 AU. This limited the velocity of the drive, although the electrical power output of a solar array at 1 AU is about 10-fold better than a nuclear power source, but rapidly decreases with distance from the sun. Assuming a 10 kW PV array, generating decreasing power out to Jupiter, the final velocity of the 1 MT craft is somewhere between 5 and 10 km/s, but with a much larger payload.

In his TVIW talk [1], Greason suggested that the 10kW power supply could propel a 2500 kg craft with an acceleration of 0.5g, reaching 400-700 km/s in just half a day. Greason [i] suggested that with this acceleration, the FOCAL mission for gravitational lens telescopes requiring many craft should be achievable. *

6. Mars Cycler

Greason suggested that the plasma magnet might well be useful for a Mars cycler, as the small delta v impulse needed for each trip could be easily met.[1]

7. Deceleration at target star for interstellar flight

For interstellar flights, deploying the plasma magnet as the craft approaches the target star should be enough to decelerate the craft to allow loitering in the system, rather than a fast flyby. Again, the high performance and modest mass and power requirements might make this a good way to decelerate a fast interstellar craft, like a laser propelled photon sail.[1]

8. Magnetoshell Aerocapture (MAC)

While the studies on the plasma magnet seemed to have stalled by the late 2000s, a very similar technology development was gaining attention. A simple dipole magnet magnetosphere can be used as a very effective aerocapture shield. The shield is just the plasma magnet with coils that do not rotate, creating a magnetosphere of a diameter in meters, one that requires the injection of gram quantities of plasma to be trapped in the magnetic field. As the magnetosphere impacts the atmosphere, the neutral atmosphere molecules are trapped by charge exchange. The stopping power is on the order of kilonewtons, allowing the craft to achieve orbit and even land without a heavy, physical shield. The saving in mass and hence propellant is enormous. Such aerobraking allows larger payloads, or alternatively faster transit times. Because the magnetoshell is immaterial, heat transmission to the shield is not an issue. The mass saving is considerable and offers a very cost-effective approach for any craft to reduce mass, propellant requirements or increase payloads. This approach is suitable for Earth return, Mars, outer planets, and Venus capture. Conceivably aerocapture might be possible with Pluto.

Figure 4. A dipole magnet creating a small diameter magnetic field is injected with plasma. As the magnetosphere impacts the atmosphere, charge exchange result in kilonewton braking forces. The diagram at left shows the craft with the training magnetosphere impacting the atmosphere. The painting on the right shows what such a craft might look like during an aerobraking maneuver. Source: Kirtley et al [3].

Making the plasma magnet thrust directional

A single magnetosphere cannot deflect the solar wind in any significant directional way, which limits this drive’s navigational capability. However, if the magnetosphere could be shaped so that its surface could result in an asymmetric deflection, it should be possible to use the drive for tacking back to the inner system.

Figure 5 shows an array of plasma magnets orientated at an angle to the solar wind. The deflection of the solar wind is no longer symmetric, with the main flow across the forward face of the array. Under those conditions, there should be a net force against the grid. This suggests that like a solar sail, orientating the grid so that the force retards the orbital velocity, the craft should be able to spiral down towards the Sun, offering the possibility of a drive that could navigate the solar system.

Figure 5. A grid of plasma magnets deflects the flow of the solar wind, creating a force with a component that pushes against the grid. If the grid is in orbit with a velocity from right to left, the force will reduce the grid’s velocity and result in a spiral towards the Sun.

Pushing the Boundaries

The size of the magnetic sail can be increased with higher power inputs, or increasing the antenna size. Optimization will depend on the size of the craft and the mass of the antenna. Truly powerful drives can be considered. Greason [12] has calculated that a 2 MT craft, using a superconducting antenna with a radius of 30 meters, fed with a peak current of 90 kA, would have a useful sail with a radius of 1130 km and an acceleration of 2 m/s2, or about 0.2g. As the sail has a maximum velocity of that of the solar wind, a probe accelerating at 0.2g would reach maximum velocity in a few days, and pass by Mars within a week. To reach a velocity of 20 km/s, faster than New Horizons, the Plasma magnet would only need to be turned on for a few hours. Clearly, the scope for using this drive to accelerate probes and even crewed ships is quite exciting.

Coupling a more modest velocity of just 10’s of km/s with the function of a MAC, a craft could reach Mars in less than 2 months and aerobrake to reach orbit and even descend to the surface. All this without propellant and a very modest solar array for a power supply.

An Asteroid, a tether and a Round Trip Flight

As we’ve seen, the plasma magnet can only propel a craft downwind from the Sun. So far I have postulated that aerobraking and conventional drives would be needed for return flights. One outlandish possibility for use in asteroid mining might be the use of a tether to redirect the craft. On the outward bound flight, the craft driven by the plasma magnet makes a rapid approach to the target asteroid which is being mined. The mined resources are attached to a tether that is anchored to the asteroid. As the craft approaches, it captures the end of the tether to acquire the new payload, and is swung around the asteroid. On the opposite side of the asteroid, the tether is released and the craft is now traveling back towards the Sun. No propellant needed, although the tether might cause some consternation as it wraps itself around the asteroid.

Conclusion

The plasma magnet as a propulsion device, and the same hardware applied for aerocapture, would drastically reduce the costs and propellant requirements for a variety of missions. Coupled with another drive such as an ion engine, a craft could reach a target body with an atmosphere and be injected into orbit with almost no propellant mass. The return journey would require an engine delivering just enough delta V to escape that body and return to Earth, where aerocapture again would allow injection into Earth orbit with no extra propellant. If direction deflection can be achieved, then the plasma magnet might be used to navigate the Solar System more like a solar sail, but with a far higher performance, and far easier deployment.

Using a steady, nuclear power or beamed power source, such a craft could accelerate to the heliopause, allowing interstellar precursor missions, such as Kuiper belt exploration and the FOCAL mission within a short time frame.

The technology of the plasma magnet combined with a MAC could be used to decelerate a slowish interstellar ship and allow it to achieve orbit and even land on a promising exoplanet.

The size of the magnetic sail can be extended with few constraints, allowing for considerably increased thrust that can be applied to robotic probes and crewed spacecraft. For crewed craft, the magnetosphere also provides protection from the particle radiation from the sun, and possibly galactic cosmic rays.

Given the potential of this drive and relatively trivial cost, it seems that testing such a device in space should perhaps be attempted. Can a NewSpace billionaire be enticed?

* These numbers are far higher than those provided by Winglee and Slough in their papers and so I have used their much more conservative values for all my calculations.

I guess we should also include a Planet Nine mission (once confirmed) among the possibilities opened up by this technology. In fact, since that planet seems to be near aphelion (otherwise we should have found it), it should also be beyond the gravitational focus of the Sun and thus any mission there could double as a “FOCAL”-type mission. Since P9 is also likely to be a gas giant, magnetoshell aerocapture after a fast transit is an exciting option.

Just one point: the sail can likely not be used to enter into orbit around an airless body (certainly not a small one), e.g., for the asteroid mining applications. So any such mission would need at least a little bit of fuel to decelerate the probe enough to enter into orbit.

Finally, what is a “2 MT craft”? 2 “metric” tons or two megatons (million tons)?

I’m guessing power is mostly expended rotating the coils against the magnetic forces in the plasma. This would be the same as with an electric motor connected to a momentum transfer mechanism, like a drive shaft with a load.

Details of the coil rotation speed were not given in the Slough papers, although the equations with that variable are given. The Winglee paper is also useful in that regard.

A couple of caveats are needed for Alex and Jeff’s enthusiasm for the concept’s applications.

Firstly, the Magnetoshell for aerobraking uses charge exchange with the neutral atoms of the atmosphere being braked against. Thus it does have an upper speed limit that depends on the atmosphere’s constituents. Best to refer to David Kirtley’s work on the concept and see for yourself.

Secondly, the coupling between Plasma Magnet and the plasma flow around it are modelled using the Magneto-Hydro-Dynamic (MHD) approximation, and the applicability of the approximation hasn’t been demonstrated over the full performance envelope described in Alex (and Jeff’s) presentations.

The reason the second point is important, is that the MHD approximation falsely gave the impression that the original M2P2 concept would achieve very high thrust efficiencies. Experimental work by the Japanese, and theoretical work from at least two different groups, have shown that the M2P2 concept was fundamentally flawed and the thrust efficiency much less.

The Plasma Magnet’s coupling to high speed plasma flows needs to be demonstrated and the thrust efficiency verified. Thus in-space experimental work is definitely required!

The coils do rotate for the plasma magnet, but are fixed for the magneto aeroshell.

If you have a reference for Kirtley regarding maximum aerobraking velocity, that would be appreciated. His work that I did see gave no obvious information on that. The implication was that a larger field would increase the aerobraking force. My assumption about the implications may well be wrong.

I used Slough’s papers for the baseline plasma magnet work. That did seem to be validated, although the thrust performance was left for the NIAC II study that never materialized. I will leave Jeff to weigh in on the high performance version that he discusses in his TVIW talk.

The Winglees’s M2P2 version was shown to be flawed, but I have not seen anything that invalidates Slough’s work. Slough notes the flaws of the M2P2 design and I thought did validate the PM design. But I’m not an expert here, and the wool could easily be pulled over my eyes.

Given the simplicity of the design, I would have thought it was quite inexpensive to test a flight version. Deploying the drive outside the Earth’s magnetosphere would settle the design issues. On that we can agree.

It is large, so it will be expensive. Also there is no reverse, so the only way to get back home to Earth is to add some kind other type of propulsion engine. VASIMR is cheaper, faster, has no turning problems and is ready to go now.

The driving coils are centimeters in size. The large diameter of the magnetosphere is due to the induced rotation of the surrounding plasma in the solar wind (or injected plasma) and the magnetic fields they induce. It is therefore cheap.

The directionality is an issue if one is a purist about drives. I have tried to show the performance benefit when used in combination with other propulsion systems.

As you know, teh issue with VASIMR is the huge power supply needed for the fast (Mars in 39 days) runs. This requires about a 2 MW power supply, usually considered to be a nuclear reactor. The mass of solar cells to produce 2 MW at 1 AU is currently going to be around 20MT and is larger by far than any previously deployed solar array in space. If you can get relatively high performance with a small power supply, with almost no propellant, and with the simplicity of some small coils, that strikes me as an elegant solution for an outward bound flight, especially so for probes that don’t require an inward bound, return flight.

This sail, coupled with a in-system main drive of a Dust Reactor array, would seem to be the path forward. The Dust Reactor generates both power and thrust and is independent of the solar wind for in-system work, while the MagSail takes us out of the solar system. Adapting this to multi-generational habitats means that we will become Sailors of the Stars, slowing into a system enough to drop off colonists and mining probes for Thorium and necessary minerals before crossing to the other side of the star and accelerating outward again. The probes, after mining and refining Thorium and whatever else, would accelerate to catch up with the Main ship, bearing the minerals necessary to continue the voyage, building new colony landers and supplies along the way.

Considering what kind of struggle the VASIMR engine has faced and how long it took for solar sails to be tested in space. I am not surprised that this engine concept’s development has stalled this decade. It would be great to get to in space tests, perhaps from a cubesat, but I realize we are years from there

Cubesats, in the hands of private institutions or individuals, might just speed up the process. Just look at what SpaceX has managed to do in such a short time. The Planetary Society is due to launch their 2nd solar sail in 2018 (I hope).

What I would like to see is the possibility of a really scaled down version of the plasma magnet that could be built into a Cubesat to test the concept. This might well be within the budget of a university group.

You should focus on what a cubesat demo of this would look like, where it would need to be deployed to (LEO or is GEO required?), and what critical aspects it would confidently demonstrate albeit at much reduced scale. This could then be submitted to one of the many peer-reviewed fora for papers, and presented at an upcoming FISO webex. The reason is that cubesats are relatively inexpensive and getting a ride is far simpler (up tp 12U). For example NASA’s Cubsesat Launch Initiative and others.

Think of the input power as affecting the size of the sail area. Slough’s equations appear to hold for quite a range of power inputs. Clearly, more power requires heavier coils and power supply, and at some point, the equations will no longer keep inflating the sail size. I’m not an expert, but it does seem to me that it is worth testing such a simple magnetic sail to determine just what its performance characteristics really are. The costs of such experiments are really the cost of space access, which SpaceX et al are trying to bring down dramatically.

Acceleration is proportional to input power for any device with a fixed thrust such as a rocket burn. You may be confusing the power to run the rocket (which is what counts from the engineers point of view) with the mechanical power the rocket has as seen by a fixed observer. These are not the same. If a rocket didn’t give a fixed delta-vee for a fixed release of chemical energy, which it does, it wouldn’t work as it does. It’s still not a ‘free energy’ device. But if you only compare the gain in kinetic energy of the rocket with the cost of expended chemical energy, that certainly is ‘over unity’. But that’s not the whole energy inventory but it is useful.

p.s. If the Woodward device does work, it gains energyfrom the interaction with the rest of the universe which is where the energy and momentum must come from. That would locally appear to be an energy generating device and one could use it as such, but it’s still not ‘free energy’ as it comes at the cost of the universe as a whole.

You seem confused by my assertion. Of course a rocket is mundane (and not overunity) for the very reason that a fixed amount of chemical energy input results in a fixed amount of delta-v; i.e. a fixed amount of kinetic energy change. I recommend you inspect the little equations I wrote down a little longer.

As Ioannis commented, these things can take a long time. Drexler was talking about solar sails in the early 1980s. Clarke edited “Project Solar Sail” that was published in 1993. Despite all this, we are just starting solar sail development, but with far smaller sails than Drexler talked about.

It would seem that a PM could be tested quite easily. If Musk took teh batteries out of that Tesla car he is launching, the saved mass could have been used to test a prototype PM drive with a solar array. I’m sure other opportunities will arise if teh concept ever gains traction.

I binge-watched every single video from that conference, and this was bar none the most thrilling of the talks. Given the affordability of the obvious next step – a test outside of Earth’s magnetosphere – I feel like this should jump to the front of the queue… maybe in the trunk of Elon’s Mars-bound convertible?

The reason why I’m so excited about it: a cheap braking system for an interstellar mission with no reaction mass. The solar system applications are pretty cool, especially the Neptune thing, but for interstellar stuff it’s absolutely a game-changer.

Greason indicated that the braking distance would still need to be quite long with such a sail. For the “short” run to Proxima, IIRC, it was 2 lys. It would make more sense for longer flights. Nowever, we are talking about a craft that is several orders of magnitude more massive than Breakthrough Starshot. As many people have noted, a larger craft can do a lot more science and solve teh communication problems that a craft massing just grams. But getting a PM probe to 0.2c is not obvious, as the solar wind is just 400-800 km/s and falls far short of that speed.

We’re glad you watched all our presentations! Preparations for the next TVIW symposium are just beginning; as soon as the dates and location are locked in we’ll pass the information on to Paul to post here. We hope to see everyone there!

I like long operational life of the system without on-board power sources, like solar sails, with unlimited resource, suitable for very long interstellar flights. But this system, like the E-sail requires on-board power source.
But now there is a lot of interesting new materials with desired properties. Did someone say something about the creation of such magnetic sails from thin fibers of very powerful permanent magnets without onboard power source? Of course, it will be impossible to roll into starting container, it will collect in the open space, but in principle, it is possible.

Solar PV arrays will give you a fairly decent lifespan for the drive. The drive itself is likely to last longer than a physical sail as there is nothing to punch holes in or degrade. The coils are small and likely to avoid damage by micrometeoroids.

Slough’s Plasma Magnet concept replaces the heavy magnet with a plasma current generating the magnetic field. The difficult to model question is whether the thrust created by interaction with the plasma flow is transferred to the space vehicle and exactly how. The experiments by Slough and the Japanese have been encouraging, but there’s significant unknowns for some applications – like braking against a relativistic plasma flow, for example.

“The difficult to model question is whether the thrust created by interaction with the plasma flow is transferred to the space vehicle and exactly how.”

That’s exactly what I’m not seeing; How does the thrust reflect back into the coils producing the rotating field, when it’s the induced dipole that actually deflects the solar wind? It would be a shame if this was just a way of blowing plasma “smoke rings” in the solar wind, and not an actual propulsion device.

What’s needed is either some really high power simulations, or an actual physical test, preferably the latter. Just another reason to be glad that the cost of reaching orbit is dropping.

This is Jeff Greason’s calculations using a larger, superconducting coil that creates a magnetosphere of over 1000km in diameter for the 2 tonne craft. His numbers work, but I am not even close to being expert enough to validate this beyond the calculations.

Alex, while my mental picture–conjured from your description–of how the Plasma Magnet Drive works is rather foggy (I’m not faulting your description; advanced “electromagnetics” isn’t something I can easily envision–this is where animated pictures, like those included in some Wikipedia articles, would come in handy!), it inspired three closely-related questions:

While the PMD can’t move faster than the solar wind (so that a “Sun-diver solar sail analog” [a ‘Sun-grazing’ PMD-powered starprobe] couldn’t get up to 10% – 12% of c), could a much larger PMD utilize cosmic rays, which–particularly the protons–move at velocities between 43% and 99.6% of c? (90% of cosmic rays are protons, 9% are alpha particles [helium nuclei], and 1% are atomic nuclei of heavier elements.) Also:

While cosmic rays arrive evenly from all directions on the celestial sphere, might a PMD utilizing a set of two conducting “hoops” in tandem, one negatively charged and one positively charged, enable a PMD to produce thrust using cosmic rays (and possibly reverse thrust at will, by changing the polarity of the charged “hoops”)? But regardless:

Even the “stock” Plasma Magnet Drive that you’ve introduced here sounds very promising, and is well worth testing in actual space flights.
Even the “non-tack-able, outward thrusting-only” basic version might–with clever use of planets’ and moons’ gravitational fields for falling back inward toward the Sun–be a very cheap, simple, and reusable solar system runabout; using thin-film solar cells (which are lighter and more radiation-tolerant) would make it robust, lightweight, and fast, and:

Speaking of conjuring images (thanks to the adjective “stock”), it might be–and I’m only half-joking here, because this concept’s simplicity and low cost likely wouldn’t require a financial juggernaut to sponsor its development–that an organization like NASCAR might sponsor PMD ships (either crewed or robotic) for racing purposes. If so, Arthur C. Clarke’s short story “The Wind from the Sun” would come *literally* true, because the ‘solar yachts’ would be propelled by the actual, physical solar wind rather by the Sun’s light…

I don’t think you can harness the cosmic rays. Outside the heliosphere in interstellar space, they need to provide both the plasma to create the magnetosphere and the directional wind to propel the craft. I don’t see how than can be translated into directional thrust.

Even more apt as a “wind from the sun” is that the solar wind is far more “gusty” than the photon “wind” and therefore some real sailing chops would make a difference in winning a race. Similarly, like the old sailing clippers, knowledge of trade winds and tides increases the performance and reduces teh times for the ships to return home with tea and spices, so might the use of known solar wind strengths and CMEs aid the trader in moving goods to teh outer solar system “colonies”.

Thank you, Alex–one possible use for cosmic rays *might* be for “trickle-power-output” alphavoltaic (as in alpha particles–protons are “hydrogen-alpha” particles) devices, which could power low-powered on-board devices or slowly charge batteries or ultra-capacitors. Also:

I think you’re right. If PMD sail spacecraft (I’m going with that term, after I just heard Jeff Greason describe it as a huge magsail in his TVIW presentation) become common, learning to “read the Sun” more intimately–to know when, where, and how strong the winds are–will likely become an important art (just as Hugo Eckener only *appeared* to fly his passenger zeppelins about at random, because he knew how to “read the sky” to pick up favorable fuel & time-saving tailwinds).

Might the PM mechanism to create large diameter magnetic fields to deflect charged particles be useful when deployed with many more to shield the Earth from CMEs? A low cost magnetic shield might save untold $ trillions in damage from fried electric utilities to destroyed data storage. The chaos from a “Carrington event” might well be worth averting by such devices. Just a speculative thought.

I thought many years ago that we could increase the size of our magnetic field by placing super conducting coils at the poles to give more protection to our satillite assets. Although not quite at super conducting temperatures there no much more energy required to make it get there.

The problem with using a magnetic shield (In space) against Carrington events, (It’s been proposed.) is the question of what you anchor the shield to. A CME carries a pretty large amount of momentum.

The damage on Earth from one of those events is due to the Earth’s magnetic field being compressed, and induced voltage in long power lines as the moving field crosses them. It’s probably a lot cheaper to just install the proper breakers to protect equipment, including utility equipment, from the voltage surge, than to build a global shield. And then accept that the grid will go down for a few hours every century or so.

Here is a proposed idea to create an artificial magnetic field around the earth. I would have though it better and a whole lot easier material and energy wise to have them at the poles as they will link up through the Earth naturally.

And that might be used for this somehow. We’ve all heard about the sundiver concept, where we send a sail to jupiter to kill off momentum–have it fall towards the Sun, then angle out to catch photon pressure.

Perhaps capture the alpha’s with a ‘Bussard collector’ and pump them into a p-B11 polywell reactor to power your ship! Would be appropriate as Dr. Bussard had the idea for both the collector and the polywell fusion reactor! ;-)

I don’t think that GCR has enough pressure to be usable, but I think that this direction pressure is achivable.
The particles are not simetrical. Protons are the most abundant and massive of the charged particles (more massive are unstable, antimatter has higher probability of previous collision, neutron is not charged, electron has less weight)
Make to magnetic bubbles to deflect protons perpendicular to your desired movement. Because there is opposite fields, protons are deflected in reverse, so it creates a pressure in your spaceship perpendicular to protons.
So, you can generate a directional force.

But It seems to me that GCR has not enough density and more inneficiencies not considered now in the model would kill this technology.

Aside from the low density of the GCR, (Thankfully!) the big problem is that they’re not going in any particular direction. It would be like trying to get thrust by deflecting air molecules, when there wasn’t any wind present. The fact that the individual molecules were moving very rapidly wouldn’t actually help, so long as their average velocity was zero.

Since the Plasma Magnet Drive can produce such a huge magnetosphere, could a “launching maser” (such as what Robert Forward suggested for his “Starwisp” maser-pushed 0.20 c sail starprobe–perhaps a “borrowed” Solar Power Satellite’s microwave beaming system) give a PMD craft any significant push? Also:

While Geoffrey Landis’ later analysis (see: https://www.centauri-dreams.org/?p=3816 ) showed that the Starwisp sail probe’s “wire cobweb-like” mesh would absorb rather than reflect the microwaves and sizzle into oblivion without going anywhere (and the scheme would also have required–for *interstellar* missions, although not for interplanetary transfers–a metal microwave fresnel lens 50,000 kilometers in diameter!), would the PMD’s magnetosphere, being a much larger microwave beam “target,” reduce the launching maser and fresnel lens size requirements (assuming that the beam *would* push against the PMD’s magnetosphere)? In addition:

A solar-powered (or perhaps nuclear-powered) charged particle beam “gun” in orbit (like those that were investigated as part of the research and development for the Strategic Defense Initiative) could propel a PMD craft–especially a small, lightweight one–particularly if its “acceleration runway” was planned so as to not be extremely long, because charged particle beams spread due to electrostatic repulsion and because they are affected by the terrestrial and solar magnetic fields. But:

A PMD craft whose “hoop(s)” could switch its/their charge polarity quickly (it’s easier to have the hoop be positive–and at a higher positive potential, of course–by using an electron gun to eject the craft’s ‘unwanted’ electrons) might begin its departure with its hoop negatively charged, with an electron beam “gun” pushing it away with the negatively charged electron beam, then later switch the hoop’s polarity to positive, in order to utilize the solar wind to produce thrust. (A positive particle beam “gun” [emitting a beam of protons, or maybe deuterons–deuterium nuclei] could be used instead [with the PMD spacecraft’s hoop always being positively charged, of course], but a positive particle beam “gun” would require more power to operate and would be more complicated and expensive than an electron beam “gun.”)

A charged particle beam should work if the beam can be kept collimated. As the particle velocity can be both high and dense, a small MP craft could theoretically ride the beam to high velocity. I suspect that the beam velocity would have to be non-constant as a near-c beam might be too fast and energetic to be deflected without a very strong magnetic field. Ramping up the beam velocity as the vehicle gains velocity might be the way to make this work. Slough ‘s proposed lab experiment to test thrust was to use a simulated solar wind in a quartz tube. I see no reason why various particle beams from accelerators could not be similarly tested.

Although I stated that the particle beam needs to stay collimated, this is not so critical for the PM. The magnetosphere expands as the local particle density decreases, maintaining a constant force. If the particle beam diverged I would expect the magnetosphere using the beam as the plasma would also expand as the beam density decreased. This is a very different behavior to a fixed size sail where a beam will spill outside the sail edges.

Thank you–I was pleasantly surprised to just hear (in his TVIW presentation) Jeff Greason speak quite favorably about using particle beams to push PMD sailcraft around the solar system–and beyond. I was particularly intrigued to hear the great power output difference between a charged particle beam (he even mentioned using charged *dust* particles!) and a laser beam that could do the same work (I knew the difference was significant, but I hadn’t expected a charged particle beam to require so much *less* power than a laser, to achieve the same final velocity!). He too mentioned the desirability of gradually “pumping up” the energy level (and thus the velocity) of the particles in the beam. Also:

Something else that he mentioned should be guarded against in all interstellar development work (and in all life, when you get right down to it). He mentioned that the MPD concept has been around for a while, but has been ignored until now for a *psychological* reason–the M2P2 concept, from which the MPD was later developed, turned out to be a poorly-performing contender, and because the two are related and were put forward by the same folks, everyone thought that the MPD, like the M2P2, was a “lemon” of a concept (my descriptive words, not Mr. Greason’s). If Jeff Greason–and now you, Alex–hadn’t picked up this ball and run it past us, the MPD would still be languishing in dim obscurity–and I wonder how many other good starflight ideas–which we still aren’t aware of–are in the same limbo state for this reason? In addition:

I have doubts about the stability of a beam of charged particles, first by electrostatic repulsion between particles of the beam and second between interactions with electrocmagnet fields existing on space. A little deviation has a huge impact on huge distances and there is a lot of changes in the medium.

I consider this model with some changes in the past. The idea is to create small “bullets”, very, very tiny and cold, to allow to maintain the direction. Neutral, with basically no charge.
To bring this neutral “beam” at speeds close to speed of light is a challenge but i think that doable. And another beam in the same direction as photons but not as a momentum transfer tool but as a energy provider tool.
The spaceship capture the energy of photons with PV panels and generate a opposite beam of electromagnetic “light” but at different frequencies to allow to the pellets to melt and turn into a charged particle bubble that it should impact into the magnetic shield and propel the spacechip.
The incoming light and spacechip PV could be changed by efficient nuclear power, fission or fusion.

Also, I noticed that Greg Matloff, at the 25:20 point (in the questions & comments portion of Jeff Greason’s PMD presentation, see: http://www.youtube.com/watch?v=0vVOtrAnIxM ), took the…not devil’s advocate, but “space colonist’s advocate” position (pointing out that Mr. Greason’s negative view regarding millennia-long interstellar transits with huge, PMD-propelled worldships was too pessimistic, because space dwellers wouldn’t regard the long journeys as being impractical [“because you live on a planet…” :-) ]), and in this connection:

If ultra-long-life (essentially immortal) interstellar probes can be developed, perhaps by the incorporation of Von Neumann-type technology to produce replacement parts on-board the probes, they could slowly roam the surrounding stars (like the fictional “stellar system fly-through” Bracewell probe called Starglider in Arthur C. Clarke’s novel “The Fountains of Paradise”), using gravity assists to travel from star to star. A propulsion system such as the PMD, which has no moving parts, would be ideal for such ultra-long-life starprobes, and it would enable them to adjust their comet-like hyperbolic “open” orbits around their target stars, in order to “set up” their long journeys to each subsequent stellar port of call.

The real problem with generation ships as a way to get someplace, is that you’re likely always going to be beaten there by somebody who went to the trouble to travel faster, even though they left later. Once we have self-replicating technology, the first arrival advantage for colonists becomes enormous, so there will be a real incentive to get there, (Wherever there is.) as fast as possible. Indeed, to get your machinery there first.

Generation ships in the form of colonies on outbound comets, on the other hand, do make a happy insurance policy for our expansion into the galaxy, as they’re likely to be sent out for motives other than interstellar colonization, but accomplish it anyway.

If this scheme works out, though, it is likely to be widely used for the real challenge of interstellar travel: Stopping at the other end of the trip!

If automated probes are it’s main use, then the plasma magnetic drive is a good idea, however it’s acceleration is too slow and it’s not maneuverable enough for manned space flight, so it’s not fair to compare it to VASIMR which does not have to get to Mars in 39 days to work. It can work on much lower power. A slower manned spacecraft will have to be able to rotate to create artificial gravity so it’s occupants will not suffer from the side effects of long term zero G exposure. Another idea to fix VASIMR might be to make some rechargeable batteries in second or third state sized containers which could be docked to the spacecraft in orbit to boost it’s power output, and speed without having to design, build and shield a radio active, space worthy nuclear powered battery.

I’m fine with VASIMR and its ilk of ion drives. The hype about VASIMR has usually been over its high power mode – requiring a lot of power. Zubrin for one has poured his usual cold water over that drive for crewed Mars missions (what doesn’t he pour cold water one that he disagrees with?).

We’ve discussed so many potential interesting deep space missions on this blog. The one major limit has been craft velocity to get anywhere quickly with a useful scientific payload. Any propuslion system that would offer high velocity with a decent payload interests me. I want to see craft reach the outer planets again, and beyond, without waiting a generation or two for results. If the PM works, as advertised, this might be one way to do that.

I’ve tried to show how the basic technology could be used in a variety of ways, from propulsion, to aerobraking to a radiation shield. Its simplicity seems very attractive to me. maybe one or more of those uses will work. If so, that is welcome progress.

PMD seems an unlikely candidate for interstellar travel when compared to our only currently-feasible option, StarShot. StarShot is designed to cruise at around 70,000 Km/s (~>0.2c) whereas Sol’s solar wind can never impart to a PMD more than about 700 Km/s, a 2 orders of magnitude difference.

Probably best to compare the PM using the solar wind with a solar sail using just the sun’s output. My reference (Vulpetti et al – “Solar Sails”) suggests very fast soolar sails might reach 100 km/s using sundive maneuvers. The PM clearly does better than that.

Breakthrough Starshot is using a laser beam propelled sail. The equivalent for the PM would be using a particle beam. That is what you would want to compare with for an interstellar mission.

Hi Alex
As Jim Benford has noted on this same web-page, particle beams have a habit of diverging excessively. The acceleration has to be extremely high to get to interesting interstellar speeds. Megagees for Starshot speeds.

Would a net neutral particle beam have reduced divergence? Accelerate the protons and inject electrons back into the beam. At least some of the beam will be neutral atoms and the rest a mix of protons and electrons.

I liked the proposal to use a particle beam of charged dust rather than ions; The higher the mass to charge ratio, the less of a divergence problem you have, and the dust could probably even be designed to shed it’s charge after being accelerated.

Now, in the presence of “stationary” charge carriers, a charged particle beam can actually self-focus rather than diverge, due to magnetic effects. But I don’t think the solar wind is dense enough for that to happen.

Due to its lower power requirements, PMD “beam sailing” (utilizing a charged particle beam “gun”) sounds like a potential candidate for “fast-enough” PMD interstellar probes. They might also be able to brake sufficiently (if not into circum-stellar orbit, at least to provide a more leisurely “fly-through” to collect data and take pictures) at their destinations, using the PMD thrust against the stars’ stellar winds (more luminous stars, like Sirius, while having stronger gravity, would also have stronger stellar winds). In addition:

Two “Centauri Dreams” comments from 2008 regarding this (see: https://www.centauri-dreams.org/?p=1665 [I’ve copied them below]) also mention this. While the technologies required for building, launching, and controlling such high-velocity, high-acceleration solar sails may still be a bit beyond our grasp (although graphene and thin-gauge niobium appear to be promising sail materials, and the occulter/launching spacecraft might be made of—or coated with—hafnium carbide in order to withstand the heat of such close solar flybys), it is encouraging to know that the Sun *could* push a properly-designed & launched interstellar sail probe fast enough to reach the Alpha Centauri system within a normal human lifetime. The light of our days is one starprobe (and perhaps starship) “engine” which, fortunately, is already available. The two comments are:

From: “Adam” January 11, 2008, 16:34

Hi Paul
Was that 500 km/s from a rocket boost or solar-sail? Claudio Maccone and Greg Matloff have computed up to 1,500 km/s for solar fry-by solar-sail missions – “The Starflight Handbook” mentions a probe design for getting up to 4500 km/s.
Utterly ridiculous star-sails could reach ~ 0.12c according to a NIAC study by one of Bob Zubrin’s colleagues.
I think I need a lie down, these speeds are getting to me ;-)

From: “Administrator” January 11, 2008, 18:02

Adam, as I recall that conversation, Geoff was talking about a pure solar sail Sun-diver maneuver — no rocket boost. I’ll double check that in my notes.

It all depends on the specific power density of the reactor. For deep space missions for robotic probes, an unshield reactor might well be very attractive. The mass quickly escalates for shielded reactors. For inner system propulsion, (Jupiter inwards) I prefer solar arrays for safety and simplicity. Solar arrays are getting thinner and with higher specific power all the time. Although old ideas of solar power for space used reflecting concentrators to heat a working fluid, the idea of using lightweight foils or Fresnel lenses to increase the output of a PV array in deep space has not been tried. O’Neill suggested concentrating solar power for his colonies out in the Kuiper belt, but those colonies would be easy to orientate to the sun. If such concentrators could be made to work, then solar power might be useful beyond their current range. Alternatively, beamed power might be the solution, whether microwave to a rectenna, or laser to the solar arrays. If Breathrough Starshot uses Lubin’s high power laser array proposal, then we might have the possibility of beaming energy to deep space probes and facilities, avoiding the need to deal with nuclear reactors near humans.

There is something exciting about magnetic field propulsion in general. To me, it evokes the magic of the sufficiently advanced, a ship moving without sound or exhaust, ethereally yoked to its reference frame.

Some questions:

Would the magnetic field’s ability to trap charged particles limit its usefulness as a shield for crew?

Am I crazy or does figure 5 demonstrate Bernoulli’s principle?

Could this be paired with an Orion themed propulsion system, with the magsail acting as a pusher plate? Coils could be arranged to form a magnetic exhaust nozzle.

The shield should work like the Earth magnetosphere. The simulations don’t show trapped particles like a mini van Allen belt, but it may do. The important thing is that trapped particles stay away from the crew.

Figure 5 just crudely represents some directional deflectionwhen an array of MP devices presents a flatter surface to the solar wind rather than an extended onion shape. It is purely speculative to suggest that the drive might be configured to allow return journeys by tacking (like a solar sail) rather than just run before the solar wind like a square-rigged sail.

Orion’s exhaust is far more energetic than the solar wind. The magnetic field strength for a ‘pusher plate” would have to be very high. The elegance of the PM is its low electrical current requirement that delivers a magnified thrust for that power.

Thanks so much for the incredible summary, it’s one of the most interesting conceptual leaps so far, like going from
generating electricity by a fixed magnet “magneto”, to generating electricity with a field coil based “dynamo”.

The discussion here raises three observation-
First, perhaps a cubesat test could utilizing the Van Allen belts as solar wind analogs?
The inner belt is dominated by protons IIRC, and isn’t that far from LEO.
This would seem to be the closest location with a knowwn flux of fast moving charged particles,
Oh, would the sail’s rotating magnetic field create a distinct proton current ring and electron current ring?

Second, given recent ideas about “sundiver” solar sails, (deploy sail in a sungrazing orbit to maximize initial thrust),
a plasma magnet sail well inside the orbit of mercury should generate some incredible accelleration forces.
Is there any way to “flicker” the field frequency / strength to modulate the force?

Third, once you have established a ring current via RMF if you turn off the RMF, how long does the ring current sustain itself?
Could it “detach” and continue, like a smoke ring? If so, could you create current rings, and then react AGAINST them?

a plasma magnet sail well inside the orbit of mercury should generate some incredible accelleration forces.

Actually not. The size of the magnetosphere decreases as the pressure of teh solar wind increases. The acceleration is theoretically constant anywhere within the heliosphere. Slough’s experiments use that result to validate the technology in the lab with a high density plasma flow in a vacuum chamber.

As regards the plasma ring current, I suspect that it dissipates once the coil fields are turned off. But that is just a guess. As for pushing against some plasma ring current, it would need to have a velocity towards the PM. I think what you may be thinking of is to have these “stationary” plasma as something to brake against. Greason suggests using a particle beam, or something similar, emitted from the destination as a braking medium for a fast Mars flight. With lower velocity, using the magneto aerobraking maneuver might be more effective, although there is no reason not to deploy different mechanisms where appropriate.

Perhaps there is a way to “rev up” a PMD within the Van Allen belts so that orbits are constrained and get up to very high speed, until the drive is decoupled from the particle field and the PMD is flung off in a predetermined direction.
Just the bare bones of a suggestion here!

Within a magnetosphere the simplest approach is just a charged tether to propel a craft to raise or lower its orbit. You would turn on another drive once you have enough velocity to leave the magnetic field. When mature, charged tethers could keep space stations in LEO at the right height to counteract the weak atmospheric drag.

This technology should be cheap enough for a private company to develop, without the need of large Government spending. Besides the obvious ground launch what is preventing this technology from being used?

It is possible to merge this technology with a nuclear pulse propulsion like Johndale Solem’s Medusa?
Medusa spacecraft used a huge parachute to convert nuclear explosion in
thrust: replacing it with a plasma magnet, we can imagine a spaceship which uses solar wind to go outward and nuclear pulse to come inward.

In a technology first, a team of NASA engineers has demonstrated fully autonomous X-ray navigation in space — a capability that could revolutionize NASA’s ability in the future to pilot robotic spacecraft to the far reaches of the solar system and beyond.

The demonstration, which the team carried out with an experiment called Station Explorer for X-ray Timing and Navigation Technology, or SEXTANT, showed that millisecond pulsars could be used to accurately determine the location of an object moving at thousands of miles per hour in space — similar to how the Global Positioning System, widely known as GPS, provides positioning, navigation, and timing services to users on Earth with its constellation of 24 operating satellites.

“This demonstration is a breakthrough for future deep space exploration,” said SEXTANT Project Manager Jason Mitchell, an aerospace technologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “As the first to demonstrate X-ray navigation fully autonomously and in real-time in space, we are now leading the way.”

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

If you'd like to submit a comment for possible publication on Centauri Dreams, I will be glad to consider it. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language, and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to your fellow readers, I'm probably not going to run it.